hostname: allow underscore
hostname: allow underscore

Issue #149

Update code to match the comment above it. Allow for underscore
in hostname as it's used quite a bit in the wild.

OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp